This invention relates to an exchanger including improved heat transfer tube assemblies through which liquid passes and is heated by a high temperature gas supplied into the heat exchanger to vaporize the liquid.
In a boiler or evaporator of a heat exchanging system, heat transfer tube assemblies comprising a plurality of finned heat transfer tubes are horizontally arranged, and liquid passing through the inner lower portion of the heat transfer tube is heated by a high temperature gas such as combustion gas or exhaust gas in a factory.
During this heating operation, vapour generated in the finned heat transfer tube assemblies flows therein above the liquid. The vapour thus obtained can be utilized for generating power to operate, for instance, turbine.
Usually a heat transfer tube assembly comprises a tube and a plurality of fins which are secured to the entire outer surface of the tube and each of which has substantially the same radial length in all directions from the peripheral outer surface of the tube. In a case where such finned heat transfer assemblies are used, heat applied to the assembly by a hot gas supplied is transferred substantially equally to the tube through fins attached thereto.
When the liquid, which may contain more or less amount of harmful chemical materials, flows in the tube, a portion of the liquid forms a thin liquid film along the upper inner wall of the tube. However, when the tube is heated by a large amount of heat, the thin liquid film is evaporated, called a dry-out phenomenon hereunder, and harmful materials contained in the liquid film will adhere to the upper inner wall of the heat transfer tube and finally corrode and damage the tube. Moreover, in an evaporator for evaporating a medium having a low boiling point and low heat stability such as Freon (Registered Trade Mark), when such a dry-out phenomenon as described above occurs, the tube is locally highly heated and the adhered medium will be thermally cracked, thus damaging the tube. For this reason, in order to avoid the corrosion of the tube due to the adhesion of the harmful materials contained in the liquid, it was required to remove beforehand the harmful chemical materials in the liquid to be heated by suitable liquid treatment means or to construct a tube with anti-corrosion material. However, the liquid pretreatment requires an additional device therefor and the anti-corrosion material utilized to construct the tubes is very expensive and uneconomical. More particularly, it is impossible to apply such expensive preliminary treatment as described above to a waste heat boiler which essentially requires a low establishment cost. Furthermore, in order to prevent the thermal cracking of the medium having a low boiling point due to excessive heating, a heat exchanger including heat transfer tube assemblies must be designed so as to reduce the heat amount to be applied to the tube to the extent that the liquid film formed at the upper inner portion of the tube would not be evaporated, but in such a case, the use of the finned heat transfer tube assemblies is limited. Moreover, a heat exchanger must be constructed to have a considerably large capacity for sufficiently absorbing the heat amount given by the high temperature gas.